Panel 1: A pair of neutron stars in a binary system spiral together. Orbital momentum is dissipated through the release of gravitational waves, which are tiny ripples in the fabric of space-time.

Panel 2: In the final milliseconds, the two objects merge and produce a gamma-ray burst lasting just a fraction of a second.

Panel 3: A small fraction of the mass of the merging neutron stars is flung out during the merger. This hot, highly radioactive material expands and its outer layer thins enough for infrared light to escape. At its peak brightness (within a week of the merger) the explosion is about a thousand times brighter than a classical nova and so is called a “kilonova.”

Panel 4: A massive neutron star or black hole remains after the event with an expanding cloud of debris around it. In addition, a powerful “wind” of material flows outward.